It is known that in the separation of a multicomponent feed in a bubble tray distillation column, bubble tray efficiency is substantially reduced by poor vapor contact distribution across the tray. Maldistribution of vapor over the tray retards tray efficiency, particularly when vapor maldistribution occurs over a lateral cross-section of the tray perpendicular to the liquid stream. In the worst case, only a portion of the bubble apertures in the lateral cross-section(s) operates to contact liquid with rising vapor, thus some liquid passes through the cross-section(s) over non-bubbling apertures thereby avoiding vapor contact.
Maldistribution of bubble contact over the bubble tray generally becomes a problem at vapor flow rates lower than a design range. Bubble tray vapor loading ranges, or "turndown ratios," i.e. the maximum to minimum vapor rates accomodated efficiently by conventional tray designs, are referenced in the literature and generally limited to the following ratios: Packed Column, 4/1; Sieve Tray, 3/1; Bubble Cap, 4/1; Valve Tray, 4/1; Dual Weighted Valve Tray, 8/1; and V-Slotted Bubble Cap with Slotted Weir, 8/1. At vapor rates below a conventional design rate indicated by the turndown ratio, a maldistribution of vapor flow over a portion of the bubble tray is generally caused by non-uniformity of the liquid pressure seal or liquid head above bubble apertures on the tray. The non-uniform liquid pressure seals are attributable to unlevelness of the tray or to the liquid level gradient inherently caused by the flowing liquid. On the other hand, vapor rates above a maximum design rate produce an undesirable excessive liquid entrainment in the vapor rising from the bubble tray.
Stepped-level bubble trays having stepped-level bubble caps or intermediate weirs have been suggested as a means to achieve a uniform liquid head by matching bubble cap elevation to the flowing liquid level gradient across the bubble tray. In this way vapor distribution is equalized across the tray because of the equalization of the pressure seal established by the liquid gradient, especially at high liquid flow loading. See "Optimum Bubble-Cap Tray Design" by W. L. Bowles, Petroleum Processing (March, 1956), page 89.
A continuation of the Bowles article in Petroleum Processing (April, 1956), page 75, suggests, for low vapor loads, blanking off some rows of caps on bubble trays, in the portion of the distillation column where lower vapor loads are anticipated. This is done for the purpose of making a single tray design usable throughout the column.
Mase, U.S. Pat. No. 1,765,087, shows a covered liquid and gas contact apparatus having baffled sections to cause gas to flow laterally in a circuitous path and make a series of contacts with liquid at a uniform velocity through the sections. The apparatus is primarily used for absorption and permits an increased number of contacts without increasing the size of the tower.
Papp, U.S. Pat. No. 3,584,844, shows circulating vapor-laden liquid from a weired bubble plate in a vertical plane to reduce a concentration gradient across the plate. The circulation of liquid decreases the specific weight of a portion of the liquid with respect to other portions in an area upstream from the end weir, the objective being uniform mass transfer conditions on the plate. In this way tray efficiency for a given flow rate is enhanced.
Koshoot, U.S. Pat. No. 3,017,950, discloses a bubble plate construction which takes the advantages of packed columns, e.g. low pressure drop, without the disadvantages e.g. poor vapor-liquid contact. The bubble plate comprises three or more plates secured in a vertical column for rotational displacement with respect to each other. To achieve uniform vapor distribution and reduce entrainment, bubble tubes carry vapor upwardly and then downwardly through the liquid. Entrainment and maldistribution are minimized since there is no liquid pressure seal above the aperatures.
However, prior art techniques do not provide for the efficient accomodation of wide range of vapor rates through the bubble tray, but rather conventional methods result in maldistribution of bubbles at the lower vapor rates or excessive entrainment at the higher vapor rates.